Extreme is Tough, more tougher then say....CoD on Veteran. The fact is, there is no regenerated heatlh, so its like Hardcore time.
Best bet is if you finish the game enough to know where all Weapons, medkits, enemies, and Boosters are.
I have to give credit to AndiakaVannir for the detail guide, but listen. There should only be 2 weapons you mainly should use at all times. ALSO keep a shotgun on the third slot through out the game, The little robots at Intreval 7 and 9 I think are killers, knocking your armor and health in just 3 seconds on Extreme, 2 shotguns close up, and there gone.
the first slot should be one of these weapons. ASP rifle is useful, but ammo is limited. Type7, This weapon should be cared when Intreval 8 is around, you can get a major stock of 70 during the underground levels of Intreval 9. Repeator should only be used at the end for the blowing up the 4 power lines at Intreval 9.
Mainly keep that Xbox Exclusive machine gun pistols at all times, till you can get ammo on ASP or Type7. Second slot should always be the Assult rifle or G2A2 and third the shotgun.
Always use the dpad for looking at corners, Aim at heads Right at SLOMO. The mechs you see, Use Proxys and Remotes to get rid of him, and Slomo. But at Intreval 6 or 7, there is a part wtih 2 mechs in a outside area, IGNORE THEM and just RUN. Saves ammo and frustration.
Also do Trial an Error, and the checkpoint system is horrible at times when it comes to some fights.
Plus pay attention to your health, Its pretty important. They take about 30 to 40 points a SHOT, Rockets about 150 and Types7, mostly instas.
Now the shotgun be useless, but trust me, IT CAN clear a room MAJORLY. most games make it like super stupid at medium range, This game, Does not. Close up is a insta and medium range, makes them stumble.
Advoid the 3 shot rocket, its uber rare to find ammo to reload on. Save Proxy for traps on harder enemies, and same goes with Remote mines. Use grenades when you have the enemy off guard and in groups, Which mostly happens intreval 3.
By any luck, WAIT FOR THE CREDITS to end, then let you hard work by happy........lets do it agian on Fear Files.